I didn't think of static electricity! that makes sense, I've tried looking inside with a mirror to locate the ground wire, but couldn't see much with the mirror I have.
I had LED lighting on dimmer switches in our jam room and had to swap them out with old fashioned incandescent fixtures and simple switches to cut out the noise. The LED's and dimmer switches were horribly noisy. Huge difference now, we can play with the lights on
I took the pickguard off the H535 and can't remember if it was noisy before I took it off???, sitting, it will occasionally make noise, not as much as standing does.
Ive used dryer sheets to cut static in other things. I'll give it a shot on the guitar